Gene expression from plasmids containing the araBAD promoter at subsaturating inducer concentrations represents mixed populations.

Gene expression from plasmids containing the araBAD promoter can be regulated by the concentration of arabinose in the growth medium. Guzman et al. [Guzman, L.-M., Belin, D., Carson, M. J. & Beckwith, J. (1995) J. Bacteriol. 177, 4121-4130] showed that expression of a cloned gene could be modulated over several orders of magnitude in cultures grown in the presence of subsaturating concentrations of arabinose. We constructed plasmids expressing a fast-folding mutant Aequorea victoria green fluorescent protein from the araBAD promoter to examine the distribution of expressed gene products in individual cells at intermediate induction levels. Microscopic examination of cells grown at low arabinose concentrations shows mixtures of brightly fluorescent and dark cells, suggesting that intermediate expression levels in cultures reflect a population average of induced and uninduced cells. The kinetics of green fluorescent protein induction suggest that this reflects an "autocatalytic" induction mechanism due to accumulation of the inducer by active transport. This mechanism, which is analogous to the induction of the lac operon at subsaturating inducer concentrations in lacY+ cells, was described 40 years ago by Novick and Weiner [Novick, A. & Weiner, M. (1957) Proc. Natl. Acad. Sci. USA 43, 553-566].